Method for producing a multifocal correction lens, and system for implementing same

ABSTRACT

The invention concerns a method for producing multifocal correction lenses from semi-finished lenses ( 1 ) having at least a first positioning marker M, associated with a correction A called distant vision correction, and a second positioning marker M′ associated with a correction B called near vision correction. The method comprises a step which consists in a surface grinding of the semi-finished lens ( 1 ) on an internal surface of said lens. The latter is inclined at a specific angle (β), for example using a wedge, so as to induce a prism of prismatic deviation such that the distant vision and the near vision optical centres are brought together and merge. The invention also concerns a system controlled by a data processing device, for example a micro-computer ( 8 ), for automatically carrying out the surface grinding step using a recorded programme and parameters input ( 80 ) by an operator.

The invention concerns the production of multifocal correction lenses,in particular optical lenses for human-vision improving glasses when thenecessary correction varies depending upon the distance of the observedobject. Such is the case for presbyopia which, as is well known, mainlyleads to lenses having a double or triple focus (so-called bifocal ortrifocal lenses), or to lenses wherein the focal distance progressivelyvaries from one point of the lens to another (commonly calledprogressive lenses). The invention in particularly concerns a method forproducing such lenses, as well as a system for implementing such amethod, i. e. in particular an automated system for producing correctionlenses, controlled by a data processing system with a recorded program.

In the prior art, it is interesting to recall that the U.S. Pat. No.2,310,925 discloses lenses of the bifocal (for distant-vision andnear-vision respectively) or trifocal type, U.S. Pat. No. 2,869,455discloses the invention of progressive lens and U.S. Pat. No. 5,430,504describes a production technology for the so-called merged lens whereinthe jump between two zones with different focuses is dimmed. Thesedocuments widely explain the method for producing multifocal lenses andthe machining performed on the front face, or convex external face.Since the curvature radius of the concave face generally is uniform,this convex face is the surface on which the different curvature radiiselected on the basis of wished powers are introduced, implying lensthickness variations. It is then assumed that the lens everywhereconsists of the same transparent, mineral or organic material.

Those lenses often are blamed for their unaesthetic aspect, resultingfrom strong thickness variations. Another group of methods dispensesfrom using a single material having the same reflection index in eachzone of the optical lens. Such methods then provide for two materialswith different reflection indices, whereby an auxiliary small-diameterlens is incorporated, by fusion, into the material of the main,large-diameter lens. This incorporation again is performed on the frontface of the main lens. The main lens is designed for distant-visioncorrection and the auxiliary lens has a complementary correction fornear-vision correction. Both corrections essentially are obtained by therelative value of the refraction indices, without requiring anydifference of the curvature radii. The variation of the global power iseasily made progressive, from one point of the lens to another, byvarying the thickness of the layers having different indices.

These lenses however are not free from inconveniences. In particular,passing from the distant-vision to the near-vision causes image jumpsthat are troublesome, and unavoidable, for the user. In an attempt toattenuate this type of inconvenience, trifocal lenses can be preferred,but again at the expense of the aesthetic aspect, due to sensiblethickness variations. The typical correction ranges, in focal distances,extend from 0,3 to 0,5 m for near-vision, from 0,5 to 1 m forintermediate vision and from 2 m to infinity for distant-vision.

The present industrial conditions in practice imply fabricatingsemi-finished lenses with various usual corrections which the industrymakes available to the opticians, so that the latter only will have toadapt the positioning of the main curvature center for each person.Those conditions furthermore tend to favor the aesthetic aspect sincethey resort to an index variation (plus potentially a progressive powervariation by surfacing) rather than a thickness variation, withouttaking into account the fact that the angular deviation between the vieworientations in distant-vision and near-vision varies from one person tothe next. More generally, nothing is done to insure an optimal usercomfort.

The main object of the present invention consequently is an improvementof the visual comfort adapted to each person, without however neglectingthe aesthetic aspect. An additional object is the respect of the optimalconditions for industrial feasibility, in particular by starting fromsemi-finished lenses such as those that presently are available, andoperating easy to use, low cost equipment.

To meet these objects in a method for producing vision correction lenseswherein near-vision correction results from a power addition as comparedto distant-vision correction, the invention essentially proposesperforming on an internal face of each lens a mechanical machining thatadds a prismatic deviation, with a reduction of the lens thickness,which prismatic deviation is calculated, based on an individual distancebetween a distant-vision application center and a near-visionapplication center, in order to bring back the optical correction centerin near-vision as close as possible to the near-vision applicationcenter.

In practice, the method implementation advantageously starts from thesemi-finished lenses in which said addition is realized, at least for amajor part, by varying the reflection index of the transparent materialmaking up said lens at the level of an external face of the lens, andthe complementary machining of the invention is then performed bysurfacing the opposite internal face.

For the most common multifocal lenses, with preferably progressive powervariation, the prismatic variation to be performed varies between 0,5and 1,5 dioptries. In the simplest case, it is applied as a singlecorrection, centered along the view motion axis between distant-visionand near-vision. It then is very easily obtained by interposing aproperly sized wedge, between the semi-finished lens and its support, inorder to bring out of center the machining axis, by spherically millingthe internal face of the lens.

To further improve the operating conditions and the industrial practiceof the invention, it often is advantageous to admit that the deviationdata to be taken into account, between the distant-vision andnear-vision centers, is the same for all individuals who have the sameaddition value for near-vision correction, for a determineddistant-vision correction value.

In a preferred embodiment for industrial applications, the inventionparticularly concerns a method for producing, in particular forpresbyopia glasses, a multifocal correction lens from a semi-finishedlens with determined optical characteristics, whereas said semi finishedlens comprises a first concave face and a second convex face andincludes at least a first positioning marker M associated with aso-called distant-vision correction A, and a second positioning markerM′, associated with a so-called near-vision additive correction B, bothlocated on said convex face and consisting of points, said method beingcharacterized in that it at least includes a surfacing step whereinmaterial is removed at a determined depth from one of said faces bymeans of abrasion machining means being translated along a first axis,in that said surfacing step includes presenting said semi-finished lensso that it faces said machining means and that a second axis orthogonalto a tangent plane at the point constituting said first positioningmarker M is inclined at a determined angle with respect to said firstaxis, so as to induce into the semi-finished lens a prism aligned onsaid rectilinear segment {overscore (MM)}′ and having an apex anglewhich is a function of said inclination angle, and in that the prismaticdeviation Δ′, in dioptries, of said induced prism complies with therelation:

Δ′=({overscore (MM)}′×A)+Y×(A+B)

where

{overscore (MM)}′ is the distance in centimeters between said points Mand M′, A and B are said corrections expressed in dioptries, and y isthe distance in centimeters between the point M′ and the opticalnear-vision center of said correction lens.

The invention will be better understood and further features andadvantages will appear when reading the following description, taking inconjunction with the appended figures in which:

FIG. 1 illustrates the principle of an addition by index variation in atraditional bifocal correction lens;

FIGS. 2A and 2B illustrate a front view and a side view, respectively,of an exemplary semi-finished lens from which the final correction lensof the invention will be produced;

FIGS. 3A and 3B illustrate two correction lenses, for a right-hand eyeand a left-hand eye, respectively;

FIG. 4 schematically illustrates a preliminary step, before surfacing asemi-finished lens, which consists of securing the semi-finished lens ona support before surfacing it;

FIG. 5 illustrates a progressive correction lens and the various opticalreferences that characterize it;

FIGS. 6A and 6B schematically illustrate exemplary bifocal correctionlenses, with and without a deviation prism, respectively;

FIGS. 7A and 7B schematically illustrate front views of a bifocal and atrifocal lens, for right-hand eye correction;

FIGS. 8A and 8B schematically illustrate two alternative surfacingdevices for producing a system of the invention; and

FIG. 9 schematically illustrates an automatized surfacing systemcontrolled by a data processing system with a recorded program.

The problem which the present invention remedies to is easily understoodwhen considering a vision correction lens for a presbyope, produced as abifocal lens as schematically represented in FIG. 1, and the positionsof the optical centers.

This lens L supposedly consists of two lenses: a main lens L₁ whichoptical characteristics defined for distant vision and a smallerauxiliary lens L₂, of a different index, that is joined on the frontface of the lens L₁ (the external face of a pair of glasses) andintroduces a necessary additive correction for near vision.

The optical centers of both lenses are not coincident but are shiftedalong a direction, here a vertical direction OY, supposedlycorresponding with the view movement direction when the user passes fromdistant vision (in principle at the center of the final lens) to nearvision (in principle downward oriented) and conversely. Punctualmarkings M and M′ respectively, corresponding with the optical centersof both lenses L₁ and L₂, are visible on the lens surface. The mark Mmaterializes the so-called application center for distant vision, andthe marker M′ the application center for near vision. Note that bothlenses cannot be physically distinguished in the case of a progressivelens. M is then defined as the center where the progression starts andM′ as the center where the progression ends.

In practice, an industrially produced semi-finished lens usuallyconsists of a circular lens 1, as schematically illustrated by the frontand side views of FIGS. 2A and 2B, respectively. The level differencewhen passing from the main to the auxiliary lens is visuallyimperceptible. The front or external face, fe, is convex-shaped with aproper curvature radius and the internal surface fi has a concavecurvature, parallel to the external face. The external face fe hasseveral markers on its surface for guiding the surface-machining step ofthe final correction lens, with a thickness reduction, according to amethod, which will be disclosed later. The figures in particularly showthe point M, the point M′ being surrounded by a small circle, an axisI_(H) designated as horizontal axis since it is perpendicular to theimaginary line joining points M and M′, and an additional marking fordistinguishing between two semi-finished lenses respectively for aright-hand eye and a left-hand eye (for instance an “R” for theright-hand eye, as illustrated in FIG. 2A).

In fact, the movement of the bearer's eye pupil normally is not strictlyvertical, when his or her view passes from one vision to the other, forinstant from a distant-vision to a near-vision. FIGS. 3A and 3Bschematically represent a pair of correction lenses, respectivelydesignated by L_(D) and L_(G) for the right-hand eye and the left-handeye of a glass-bearer. The meaning of the references M_(D), M_(G),M′_(D) and M′_(G) is the same as for the above references M and M′, butthey are associated to the right-hand eye and the left-hand eye,respectively. Orthonormal XY axes centered on points M_(D) and M_(G),respectively, are represented on the correction lenses L_(D) and L_(G).If the pupil centers of both eyes are projected on the vertical axis Y,respectively in P_(D) and P_(G), it easily is ascertained that bothrectilinear segments M_(D)-M′_(D) and M_(G)-M′_(G) are inclined,oppositely with respect to the vertical axis Y. In the trigonometricdirection, the rectilinear segment M_(D)-M′_(D) is at an angle −α_(D)with the vertical axis M_(D)Y and the rectilinear segment M_(G)-M′G atan opposite angle +α_(G) with the vertical axis M_(G)Y. Those anglesα_(D) and α_(G) usually have the same absolute value, in the order of 7to 8 degrees.

Reverting now to FIG. 1, this shows a point O′ which materializes whatmay be called the optical correction center for near vision, by analogywith a point O (not represented since supposedly coincident with pointM) that constitutes the optical correction center for distant vision,proper to the main lens. Due to the usual lens production conditions,the location of point O′ is intermediate between M and M′. For a goodquality near-vision, it however is desirable, as provided by theinvention, that points M′ and O′ be coincident or at least very close toone another, such that the view stays centered on the optical center ofthe used correction zone.

According to the laws of optics, the following relation holds truebetween the distances MO′ and MM′: $\begin{matrix}{\overset{\_}{{MO}^{\prime}} = \frac{\overset{\_}{{MM}^{\prime}} \times B}{A + B}} & (1)\end{matrix}$

where A represents the distant-vision correction along the MM′ axis andB the additive correction for near vision, with both correctionsexpressed in dioptries. By convention, the positive direction of thevectors is from top to bottom. The above relation clearly shows that thevalue {overscore (MO)}′ normally is not zero. The larger thedistant-vision correction, the larger the distance between O′ and M′.This results in near-vision deformations, which cause troubles,potentially even nausea, for the user. This inconvenience can beremedied to, in particular in progressive lenses, by creating aninduced-prism effect, as will now be disclosed.

As already indicated previously, a correction lens is produced bymachining a semi-finished lens (see FIGS. 2A and 2B) that advantageouslywas selected, in a standard range, on the basis of the amplitude of thecorrections to be obtained. As schematically illustrated in FIG. 4, thesemi-finished lens 1 is arranged on a support 2 that includes asubstantially cylindrical main body 20 topped by a ring-shaped crown 21acting as a receptacle for the external face (convex in the example ofthe FIG. 4). The semi-finished lens 1 is adhesively blocked by means ofan easily melted metal.

The positioning is performed with the help of markings on the surface ofthe external face fe (see FIG. 2A). For this purpose also, athrough-running channel 22, with an axis A_(H), may be provided in thecylindrical body 20 and the ring-shaped crown 21 Point M can thus beseen both from front and from behind and be positioned at the center ofthe opening of this channel 22

According to an additional arrangement, a wedge-shaped shim 3 isinserted between the external face fe and the crown 21 in order toinduce a prismatic optic deviation into the finally obtained lens. Thereconsequently is an angle β between the axis A_(H) and the axis A′_(H)that is orthogonal to the tangent plane in M to the surface of theexternal face fe. It should be noted here that the shim 3 in practice isnot a full object. It preferably is materialized by three tips such thatadjustable movements thereof are controllable to modify the orientationand angle of the prism.

In order to produce the final correction lens, a machining is thenperformed by surfacing the internal (concave) face. The assemblyconsisting of the lens 1 and support 2 is presented to a(non-represented) machine tool, with the support being locked in areception member and a priori being movable along the axis A_(H). Sincethe lens 1 in inclined with respect to the axis A_(H), the desired prismis reproduced during the machining operation, with an apex angledepending upon the shim angle, but in the opposite direction.

According to the invention, the value of the added prism is calculatedso as to optimize the position of O′. This allows guaranteeing both agood compatibility with the currently available technological means inthe optical industry, and an instantaneous and comfortable near-visionreading, independently from any distant vision correction and anynecessary addition, in particular for progressive lenses. Since thereading application center directly is at its ideal position, any effortin searching for it can be avoided. All near-vision deformations arevery strongly attenuated and the intermediate visions are instantaneous.Passing from distant vision to intermediate and/or near vision isperformed without any image jump for all correction lens types, eitherbifocal, or trifocal, or with a progressively variable correcting power.

In the case of progressive lenses, it may be admitted that the eyerotation angle, in an angular view movement, substantially is constant,typically in a range from 37 to 38 degrees. This allows adoptingparticularly advantageous implementations of the invention. Theimplementation of the method of the invention will now be described in amore detailed manner, while referring to FIG. 5 which shows a front viewof an exemplary progressive-type correction lens.

The main characteristically markings of this lens L are drawn on thisFIG. 5:

M: center where the progression starts, which also will be calledblocking center since it is used as a reference for positioning thesemi-finished lens (see FIG. 4);

M′: end of progression;

O: optical center in distant vision;

O′: by analogy, optical center of the semi-finished lens in near vision(in fact a combination of near vision and distant vision);

O″: optical center, again in near vision, but taking into account thepresence of an induced prism along the axis MM′, a characteristicfeature of the invention;

A: distant-vision correction along the reference axis MM′;

B: additional correction value (near vision)

Δ′:prismatic deviation added to the correction lens, expressed indioptries;

α: angle between the axis MM′ and the vertical Y-axis of a orthonormalreferential system XY.

In a normally surfaced lens, i.e. Without any prismatic correction,points O and M are coincident and the position of O′ is given by therelation (1). However, for a good quality near vision, points M′ and O′need, according to the invention, to be coincident or at least veryclose to one another. In order to obtain this result, the inventionprovides for adding a vertical prism to the lens during the surfacingstep. The prism should have a lower basis, i.e. Be positive, for a lensassociated with a positive correction A; and it should have an upperbasis, i.e. be negative, for a lens associated with a negativecorrection A. A particular case happens when A=0 In this limit case, noadditional prism is needed.

In order to clarify the explanation, FIGS. 6A and 6B schematicallyillustrate examples of bifocal correction lenses L_(DF) and L′_(DF)respectively without and with a vertical prism. For illustrationpurposes, the figures are not drawn to scale, in order to betterevidence the prismatic configuration in the correction lens L′_(DF) ofFIG. 6B.

When a vertical prism is introduced, this introduction results in pointO′ becoming O″. When referring to relation (1), the following relationmust hold true for O″ and M′ to be coincident:

{overscore (MO)}″={overscore (MM)}′  (2)

The laws of optics, which translate the prismatic deviation of any prismby the relation:

Δ=D×d  (3)

where D is the power in dioptries and d the distance in centimeters,allow expressing the prismatic deviation Δ of the inventive correctionlens (for instance L′_(DF) of FIG. 6B) as a function of {overscore(MM)}′, B and A to cause points O″ and M′ to be coincident.

The following relation is obtained:

Δ′={overscore (MM)}′×A  (4)

In this case, when the optical power is expressed in dioptries and thedistances in millimeters, the position of O becomes: $\begin{matrix}{\overset{\_}{MO} = {\frac{10\Delta^{\prime}}{A} = \frac{10 \times \overset{\_}{{MM}^{\prime}} \times A}{10 \times A}}} & (5)\end{matrix}$

hence

{overscore (MO)}={overscore (MM)}′  (6)

There follows that both optical centers O and O″ are coincident. Forthis reason, it can be guaranteed that there is no image jump ormovement. The visual comfort also is optimized since M′ and O″ arecoincident and there consequently is no deformation in a near-visionimage.

A preferred embodiment of the invention for a safe industrial practice,however, only includes machining the internal face for reducing the lensthickness and consequently acting on the basic lens power fordistant-vision, without modifying the addition zone for near-vision asfigured by the index variation of the material on the external face ofthe lens. A single prism effect that will be the same all over the lenswill consequently be added. The calculation of the thus inducedprismatic deviation furthermore can be still simplified by admittingthat the distance between the application center in near-vision and theapplication center in distant-vision on the lens is the same for allpersons requiring the same correction (distant-vision plus near-visionaddition) with a same type of no multiple or progressive lens.

Supposing that O″ should be moved over a distance y with respect to M′,along a vertical direction or, more precisely, along the axis MM′ (FIG.5), a prism Δ′ should be provided with a power complying with to thefollowing relation:

Δ′=({overscore (MM)}′×A)+Y×(A+B)  (7)

where A and B are expressed in dioptries and the distances are expressedin centimeters. If y is sufficiently small, the results thus obtainedare very close to those obtained when the relation (4) is complied with.The relation (7) consequently is the more general relation, while therelation (4) strictly is verified when y=0.

To clarify the explanation, the implementation of the method of theinvention will now be detailed for three particular cases of lenses,namely with bifocal lenses, trifocal lenses and progressive lenses.

FIG. 7A schematically illustrates a front view of a bifocal lens 4 for aright-hand eye correction. It includes two distinct zones: the main lens40 and a so-called “chip”, i.e. a small zone 41 that constitutes thenear-vision zone. Both points M and M′ located in the zones 40 and 41respectively also are represented on this FIG. 7A.

If A_(OD) and A_(GO) are the corrections to be reached for theright-hand eye and the left-hand eye, respectively, and if theinclination α of the axes MM′ supposedly is +8 degrees and −8 degreeswith respect to the vertical, then A_(OD) and A_(OG) traditionallycomply with both following relations (where the angles are expressed indegrees and A_(OD) and A_(OG) in dioptries):

A_(OD)=SPH+CYL cos (γ−8)  (8)

and

A_(OG)=SPH+CYL cos (γ+8)  (9)

where γ is the astigmatism axis angle, SPH is the value of thecorrection lens sphere and CYL is the astigmatism value of thecorrection lens.

In this case, the prism to be added according to the method of theinvention typically is given by the relation

Δ′=A  (10)

for a distance MM′ usually equal to 10 mm.

FIG. 7B schematically illustrates a front view of the trifocal lens 5for correcting the right-hand eye vision. It comprises three distinctzones: the main lens 50 and two small superimposed zones 51 and 52 forthe intermediate vision and the near vision, respectively. Aspreviously, both points M and M′ located in the zones 51 and 52,respectively are represented on this FIG. 7B.

In the case of trifocal lenses, the average distance between M and M′typically is 16 mm. The relation (10) consequently becomes:$\begin{matrix}{\Delta^{\prime} = \frac{16A}{10}} & (11)\end{matrix}$

The method of the invention also applies to progressive lenses. Those infact constitute the preferred embodiment of the invention since theadvantages obtained in both an improved visual comfort and anappreciated aesthetic aspect are particularly substantial here, whilerespecting the industrial feasibility. The prismatic deviation added tothe traditional semi-finished lens, calculated as a function of thedistance MM′ (which represents an individual angular spacing betweennear vision and distant vision) and the power addition between distantvision and near vision, while bringing O′ (the optical center for nearvision) closer to M′ according to the invention, also cause the opticalcenter O to be more distant from the point M. This however in practiceonly exerts a perfectly negligible incidence on the visual comfort, dueto the extent of the visual fields since the near-vision correction (3dioptries for instance) for most presbyope is substantially larger thanthe distant-vision correction (0.5 to 1 dietary in the other direction).

Such lenses already are represented in FIGS. 3A and 3B Reverting to theexample of FIG. 3A (right-hand eye correction), the projection of thepupil center on the lens is represented in P_(D). The distance betweenP_(D) and M_(D) equals 2 mm, on the average. The distances between M_(D)and M′_(D) and between P_(D) and M′_(D) respectively are 14,5 and 16,5mm. M′_(D) is 2 mm shifted towards the inside. The 16,5 mm distancebetween the points P_(D) and M′_(D) corresponds to a vertical eye anglerotation in the order of 37 or 38 degrees, to pass from the distantvision to the near vision. The same value will naturally be found forthe left-hand correction lens eye (FIG. 3B: LG).

The axis M_(D)M′_(D) substantially is vertical, similar to the case ofbifocal and trifocal lenses, but with a slightly larger deviation angle,typically 12 degrees, with respect to the vertical. The relations (8)and (9) then become:

A_(OD)=SPH+CYL cos(γ−12)  (12)

and

 A_(OG)=SPH+CYL cos(γ+12)  (13)

The ideal value for Δ′ typically is given by the following relation(29=2×14,5 mm): $\begin{matrix}{\Delta^{\prime} = \frac{29A}{20}} & (14)\end{matrix}$

For a {overscore (M′O″)}=y shift (in millimeters), the value of theprism Δ′ consequently is given by the relation: $\begin{matrix}{\Delta^{\prime} = \frac{{29A} + {2{y\left( {A + B} \right)}}}{20}} & (15)\end{matrix}$

The complete method for producing correction lenses of the inventionwill now be described. As reminded earlier, the lenses will be producedfrom commercial semi-finished lenses available from various companies.Producing those lenses is not directly within the scope of theinvention. The lens-surfacing step to obtain the features of theexpected result, notably to comply with the relation (4), is perfectlycompatible with the technologies used in the prior art, which offers adefinite advantage.

According to a first method of the invention, as described in referenceto FIG. 4 which now is reverted to, a semi-finished lens 1 (see FIGS. 2Aand 2B) is positioned on a support 2 having a body 22 and a ring-shapedcrown 21 for receiving the semi-finished lens 1. As already indicated,the positioning is performed with the help of the visible markings onthe convex surface (FIG. 2A: fe) of the semi-finished lens 1. In orderto obtain the required prismatic deviation value Δ′, a prismatic insert3 (prism Δ) is introduced according to this embodiment between thesemi-finished lens 1 and the support 2.

For bifocal and trifocal lenses, the prism value Δ is identical to thevalue Δ′. This prism value consequently complies with the relation (10)or (11) for a bifocal or a trifocal lens respectively.

For progressive lenses on the other hand, the induced prism on the lensΔ′ differs from the prism physically represented by the shim Δ. Whereveran approximation corresponding to the most frequent cases of a presbyopeis not considered as satisfactory, corrections should be added for Δ′ tocomply with the above relation (15), when the dimensions of the insertedshim (FIG. 4:3) are taken into account. Practice shows that establishinga mathematical formula describing the above-mentioned corrections for acorrelation between the shim dimensions and the induced prism isinappropriate, and an experimental calibration is preferred to determinesuch correlation.

The first step consists in determining a so-called “rough” shim valuefor the prismatic shim, supposing that Δ=Δ′. Physically, the apex angleof the prismatic shim is equal to the apex angle of a prism equivalentto Δ′. Corrections then must be added in a second step for reaching thefinal expected results. These corrections are experimentally determined,for instance by performing comparisons on prototypes produced withvarious addition values B and a set of predetermined prismatic shims,for Δ>0 and Δ<0. No addition is needed if the prism value is Δ=0, aspreviously noted. Furthermore, obtaining is equivalent to inserting oneshim in the positive direction and two identical shims in the negativedirection. This operating mode simplifies the shim insertion process.The above mentioned corrections allow refining the results and obtaininga convergence between the finally obtained prismatic corrections and thedesired value, i.e. The value complying with the relation (14).

After performing these preliminary operations, the final correction lenscan be obtained by a surfacing method compatible with the traditionalmethod of the prior art.

FIG. 8A illustrates one of the currently used methods. A so-calledsurface generator is used as a machine tool 6. It includes a millingcutter 60 such that the diameter of its abrasive front face 62advantageously is substantially equal to or larger than the diameter ofthe semi-finished (commonly circular) lens 1 and the curvature radius isequal to the radius of the convex face fe. The body 20 of the support 2of the semi-finished lens 1 is locked by jaws 63 or any similar member,of a fixed support (not represented), mechanically coupled with themachine tool 6. The milling cutter 60 is placed at the end of arevolving shaft 61, the symmetry axis of which is coincident with thesymmetry axis A_(H) of the support 2. When the milling cutter istranslated along this axis A_(H), it will machine the concave face fi ofthe lens 1. Since this axis is slanted at an angle β with respect withthe axis A_(H), the surfacing process causes both a material removal andthe creation of a prism in the lens, in a direction opposite to the shim3. The action on the lens 1 is continued, in a manner well known per se,until a predetermined correction lens thickness is obtained.

An exemplary milling cutter 60 is made of diamond material and typicallyrotates at a speed of 4500 rpm.

The following step, in a manner well-know per se, consists of polishingand buffing both surfaces fe and fi potentially, but not necessarily,after having brought them back in a normal position, i.e. Wherein thelens is not slanted, in an identical manner for both surfaces. Thoseoperations do not significantly modify the correction values obtained inthe surfacing step. Lens surface treatments, such as an anti reflectiontreatment, also can be performed on the external face fe.

The lens finally is cut according to a predetermined template, again ina well-known manner per se. The final lenses are not necessarilycircular. The correction lenses, for both eyes, are cut according to thelens mount that will hold them.

The just described surfacing method introduces a physical prismaticshim, that will induce into the lens an opposite prism of a strictlyidentical value for bifocal or trifocal lenses or an approximate valuefor the progressive lenses. The same effect can be obtained withoutintroducing any prism. The same effect namely is obtained if thesemi-finished lens has a symmetry axis coincident with the axis of thesupport 2 and if the milling axis is slanted with respect to thesymmetry axis.

FIG. 8B schematically illustrates this milling method. The shaft 61 thatsupports the body of the milling cutter 60 rotates around an axis A″_(H)which is at an angle β with respect to the axis A_(H). The device ofFIG. 8A is perfectly dual of the device of FIG. 8B. It must be clearlyunderstood that this concerns a relative slant of both axes A_(H) andA″_(H) and that the latter potentially may stay horizontal. It namelycan be simpler to slant, in an appropriate manner, the support holder 63rather than the rotating shaft 61 of the machine tool 6. Both methodsalso can be combined.

Finally an other known way consists of locking the semi-finished lenswith the help of three tips solitary with the support, at least one ofwhich has a length different from the others. There follows that thesemi-finished lens is supported by a tripod and, as previously, ispresented in a slanted manner to the milling cutter. If the lengths ofall three tips are equal, an alternative similar to the alternative ofFIG. 8B can be implemented.

Still other methods also are well known, in particular a method thatresorts to a small milling cutter and scans the whole surface of thesemi-finished lens. This method generally gives less precise results andthe milling cutter will very rapidly wear off.

A preferred embodiment includes a possible complete automation of boththe semi-finished lens machining step and the step of producing theprism with the predetermined value (i.e. Complying with one of therelations (4) or (7), in general, and one of the relations (10), (11),(14) or (15), in particular, depending upon the type of correction lensto be obtained).

FIG. 9 schematically illustrates a complete system allowing such anautomation.

A first rotating motor 64 drives the shaft 61 that supports the millingcutter 60. The support 66 for this motor is mechanically coupled to asecond rotating motor 68, for instance by means of a gear including aworm or a rack 67, (or any similar device), that will drive the support66 along an horizontal axis A_(H). A step motor also can be used insteadof the rack 67 and the rotating motor 68. The system finally includes asliding way type device affixed on a planar support (not represented) orany similar device for guiding the horizontal translation of the motor64 and holding this motor.

In the exemplary embodiment described in FIG. 9, the semi-finished lens,with a symmetry axis A′_(H), is attached with a support, referenced hereby 2′. The support 2′ itself is supported by a motorized positioningapparatus. It is spatially positioned in such a way that the point M ison the horizontal axis A_(H) (the horizontal axis and the symmetry axisof the shaft 61) and that the symmetry axis A′_(H) of the semi-finishedlens 1 is at a predetermined β angle with the axis A_(H). This angle βis such that the value of the induced prism Δ′ that complies with one ofthe previously mentioned relations will be obtained. In order for thosetwo requirements to be simultaneously satisfied, the support 2′ needstwo degrees of freedom: namely a rotation around an horizontal axisorthogonal to the axis A_(H), for obtaining the inclination angle β, anda translation along the axis A′_(H) to place the point M on the axisA_(H).

The various motorized members are controlled by a data processing system8, with a recorded program, which system, for instance includes ageneral purpose microcomputer with one or several (not represented)specific cards, with appropriate input and output ports where thevarious motorized members will be connected through specialized orstandard connections (parallel, serial).

In FIG. 9, the data processing system is a microcomputer 8 with standardperipherals, in particular a display 81, a keyboard 80 and a diskettereading unit 82. The figure also represents the main connections betweenthe microcomputer 8 on the one hand and the motorized members 7, 64 and68 on the other hand.

The connection I₁ transmits commands to the member 7 for controlling thepositioning of the support 2′, in both rotation and translation. Thissupport is associated with one or several traditional sensors, inparticular (not represented) position sensors, for instance of theonto-electronic type. Such sensors allow determining, among others, thespatial position of the semi-finished lens 1. Markings (see FIG. 2A) onthe surface fe of the semi-finished lens 1 can be used for this purposesince the exact position of the fixed horizontal axis A_(H) is known. Anoptical reading of the spatial positions of those markings can inparticular be performed.

An additional connection I₂ transmits to the microcomputer the result ofthe performed measurements, no matter how they were performed. Themicrocomputer can then exert, via the connection I₁, a real time controlof the movement of the semi-finished lens 1 such that the abovementioned positioning requirements are satisfied, and it can block thelens in a position proper for presenting it to the milling cutter 60 atthe required inclination angle β. Both connections I₁ et I₂ naturallycan be grouped into a single bidirectionally connection.

The microcomputer 8 controls the operation of the motor 64 through theconnection I₃. This can imply either simple on-off commands, or alsocommands for controlling the rotation speed of the motor 64.

The microcomputer 8 finally controls the back-and-forth translation ofthe milling cutter 60, along the axis A_(H), by means of the motor 68and the worm 67 acting on the base 66 of the motor 64 (in the describedexample). A connection I₅ transmitting the back-and-forth motioncommands to the motor 68 is provided for this purpose. A (notrepresented) position sensor that at any time transmits data definingthe position of the milling cutter 60 also is needed. It can be anelectro mechanical transducer or an optoelectric transducer; codedwheel, etc., coupled with the worm 67.

If the motor 68 is of the stepper type, digital data representing theposition of the actuator acting on the basis 66 of the motor 64generally are available. Such data can be directly used by themicrocomputer 8, without requesting any analog to digital conversion.Connection I₄ transmits the measurement signals for the position of themilling cutter 60 along the axis A_(H). Here again, both unidirectionalconnections I₄ and I₅ can be grouped into a single bidirectionallyconnection.

It should be clear that. All connections I₁, I₃ and I₄ for controllingthe motorized members 7, 64 and 68, do not normally transmit anyelectric power signals but only act upon electromechanical switches(relays etc.) And/or electronic switches (semi-conductor switches, etc.)Arranged between the traditional electrical and/or fluidity supplycircuits (not represented) and those motorized members.

The microcomputer 8 records, in the mass memory (hard disk, notrepresented) which it is usually is equipped with, all data and programcommands for the production of correction lenses, in particular for thesurfacing machining step. It even more practically records, according tothe main characteristics of the invention, the data and commandsnecessary for obtaining the prism induced into the lens and complyingwith the relation (4), in a general manner, or with any of the specificrelations (10), (11) or (14) in a more particular manner, depending uponthe type (bifocal, trifocal or progressive) of the correction lens to beobtained.

In the case of progressive lenses, since the induced prism obtained,with a value Δ′, cannot here be directly directed derived from the valueof the angle β, it also is useful to record a data base of theexperimental results for a range of prototypes produced with variousvalues for angles β and additions B. These data are then used tointroduce the above mentioned corrections.

The data and program commands can initially be input, either manuallythrough the keyboard 80, or preferably by reading a diskette DK(diskette reading unit 82) or any other magnetic or optical support, aslong as the microcomputer 8 has an appropriate reading unit. The dataand commands also can be downloaded into the microcomputer, via a modem.This arrangement is particularly advantageous if the correction lensproduction site is in a store that is part of a store chain. Theprograms and applicative data can then be elaborated in a centralizedmanner and be made available in real time, simply by interrogating acentral data base available to all subscribers, either for producingcorrection lenses, or to initialize and update a local data base.

Those skilled in the art will directly understand that the recordedcontrol programs and/or data associated therewith are easily modified orupdated, in order for instance to take into account the availability ofnew types of semi-finished lenses, or more simply to correct errors inthe program or improve the performances. Such modifications also arecompulsory when another machine tool is used or when some components ofthe machining chain are replaced. This feature adds to the flexibilityof the method.

In an operational functioning mode, an operator will input the necessaryparameters for implementing the surfacing step on the correction lens tobe realized, while taking into account all parameters associated withthis step: features of the basic semi-finished lens, type of correctionlens and values of corrections to be obtained (A, B), angular deviationβ to obtain the prism Δ′, potentially correction parameters forprogressive lenses (or at least an indication that such correctionshould be introduced to insure that the program then automaticallyintroduces them). The data and commands being input will be displayed onthe screen 81 in a text and/or graphics form. As an example, the programcan display a menu including questions which the operator will answerfor entirely defining the correction lens he or she wants to produce.The program can in response display on the screen 81 the features or themodel of the semi-finished lens that should be used, if those data wereinput at the previous step.

The operator can then initiate the surfacing step proper, which isautomatically performed under control of the recorded program which heor she has parameterized at the previous step. This surfacing step isperformed in the previously described manner, with big-directional dataand/or commands being exchanged through the various connections I₁ to I₅between the microcomputer 8, the motorized members 7, 64 and 68 which itcontrols, and the sensors, in particular the position sensors associatedwith those motorized members.

Other types of supports naturally can be used for the semi-finished lens1, for instance a three-tip type support. All that is required is thatthe symmetry axes of the semi-finished lens 1 and the milling cutter 60are at a determined angle β, so as to induce into the lens a prism thatcomplies with the relation (4), which allows the points M′, O′ and O″ tobe coincident, or almost coincident, when the relation (7) is verified.

The method also is compatible with the production of a correction lensesfor astigmatism by using the relations (8) and (9) or (12 and (13).

In the light of the preceding description, those skilled in the art willclearly see that the invention does reach its object.

The producing method, comprising a surfacing step, allows obtainingmultifocal, in particular bifocal, trifocal and progressive correctionlenses, and offers many advantages. Those correction lenses, inparticular do not cause the disagreeable phenomenon of image jump whenpassing from one vision mode to another (distant vision to near vision,for instance). The near-vision deformations are imperceptible. Thoselenses offer both a substantial reading comfort and an instantaneousadaptation.

The production method is compatible with prior-art technologies andallows using, as the basic material, currently available semi-finishedlenses selected in a standard range.

And the method, according to a preferred alternative embodiment, finallyallows a large process automation by using computerized means.

It however should be clear that the invention is not limited to the onlyexemplary embodiments explicitly described above, in particular inrelation with FIGS. 5 to 9. Other technologies, of which only some weredetailed, can in particular be used for the surfacing step.

What is claimed is:
 1. A method for producing vision correction lensesfrom semi-finished lenses having a power addition for near-visioncorrection with respect to distant-vision correction, wherein amechanical machining that adds a prismatic deviation is performed on aninternal face of each lens, through reducing the thickness thereof, andthis prismatic deviation is calculated, based on an individual distancebetween a distant-vision application center and a near-visionapplication center, in order to bring back the optical correction centerin near-vision as close as possible to the near-vision applicationcenter.
 2. The method of claim 1 for producing, in particular forpresbyopia glasses, a potentially progressive, multifocal correctionlens from a semi-finished lens with determined optical characteristics,by varying the index and/or thickness, wherein, said semi finished lenscomprising a first concave face and a second convex face and includingat least a first positioning marker M associated with a so-calleddistant-vision correction A that materializes said application centerfor distant-vision, and a second positioning marker M′, associated witha so-called near-vision additive correction B that materializes saidapplication center for near-vision, said method includes at least asurfacing step wherein material is removed at a determined depth fromone of said faces, preferably said internal face, by means of abrasionmachining means being translated along a first axis (A_(H), A″_(H)),said surfacing step including presenting said semi-finished lens so thatit faces said machining means and that a second axis (A′_(H)) orthogonalto a tangent plane at the point constituting said first positioningmarker M is inclined at a determined angle (β) with respect to saidfirst axis (A_(H), A″_(H)), so as to induce into the semi-finished lensa prism aligned on said rectilinear segment {overscore (MM)}′ having anapex angle which is a function of said inclination angle (β), andwherein the prismatic deviation Δ′, in dioptries, of said induced prismcomplies with the relation: Δ′=({overscore (MM)}′×A)+Y×(A+B)  in which{overscore (MM)}′ is the distance in centimeters between said points Mand M′, A and B are said corrections expressed in dioptries, and y isthe distance in centimeters between the point M′ and the opticalnear-vision center of said correction lenses.
 3. The method of claim 1,wherein said semi-finished lens is selected in a standard range for saiddistant-vision correction A and consists of a substantiallycircular-shaped glass block having determined optical characteristics,with said block being inscribed between two faces having identicalcurvature radii, one of which is a convex, so-called external face andthe other is a concave, so-called internal face, and wherein saidmechanical machining comprises a surfacing step performing a materialremoval from said concave face at a determined depth, with thesemi-finished lens being presented to machining means at an inclinationangle (β) including said prismatic deviation.
 4. The method of claim 3,comprising a preliminary step consisting of: securing said semi-finishedlens on a support having a symmetry axis, placing between said supportand said semi-finished lens a prismatic shim with an apex angle equal tosaid inclination angle (β), but in a direction opposite to said inducedprism to be produced, so as to obtain a same inclination of thissemi-finished lens with respect to said symmetry axis, and placing saidsupport so that it faces said machining means with said symmetry axisbeing coincident with a translation axis of said machining means.
 5. Themethod of claim 2, including a preliminary step consisting of: securingsaid semi-finished lens on a support having a symmetry axis (A_(H)) sothat this symmetry axis (A_(H)) is coincident with said second axis(A′_(H)), and placing said support so that it faces said machining meansand that said symmetry axis (A_(H)) is at an angle equal to saidinclination angle (β) with said translation axis (A″_(H)).
 6. The methodof claim 3, including, when said correction lens to be realized is ofthe bifocal type, a step of selecting a value for the inclination angle(β) corresponding to an apex angle of the induced prism such that theprismatic deviation of said prism Δ′ is equal to said distant-visioncorrection A expressed in dioptries.
 7. The method of claim 3,including, when said correction lens to be realized is of the trifocaltype (5), a step of selecting a value for the inclination anglecorresponding to an apex angle of the induced prism such that theprismatic deviation of said prism complies with the following relation$\Delta^{\prime} = \frac{16A}{10}$

where A is said distant-vision correction expressed in dioptries.
 8. Themethod of claim 3, including, when said correction lens to be realizedis of the so-called progressive (L) type: a first step of selecting aso-called rough inclination angle value (β) equal to the apex angle of aprismatic deviation prism that complies with the relation:$\Delta^{\prime} = \frac{29A}{20}$

where is said distant-vision correction expressed in dioptries, and asecond step of correcting said rough angle value with the help ofexperimental data so that said prismatic deviation of the prism inducedinto the lens will converge towards said relation$\Delta^{\prime} = \frac{29A}{20}$

where said experimental data is obtained from comparative measurementsof a series of progressive-lens prototypes with determined prismaticdeviation corrections and additive corrections.
 9. A system forproducing a multifocal correction lens, in particular for glasses, byimplementing a method according to claim 1, comprising: a support forsaid semi-finished lens with inclination means for slanting said supportwith respect with a horizontal axis (A_(H)), machining means comprisingan abrasive milling cutter for a said concave face of the semi-finishedlens, wherein said machining means comprise a rotating motor drivingsaid abrasive milling cutter at a determined angular speed, andmotorized means for translating said milling cutter along saidhorizontal axis (A_(H)) so as to implement a surfacing step removingmaterial from the lens up to a determined depth in said concave face.10. The system of claim 9, wherein said inclination means of thesemi-finished lens are motorized, wherein said system includes positionsensors, for measuring the spatial instantaneous positions of saidmotorized inclination means, said abrasive milling cutter and saidtranslating motorized means for said abrasive milling cutter, and a dataprocessing system with a recorded program, including at least means forinputting and collecting data and program instructions and storing thesedata, and wherein said data processing system controls said motorizedinclination means, said rotating motor and said translating motorizedmeans for said abrasive cutter, based in particular on said measurementsby said position sensors and on parameters collected via said datainputting and collecting means by an operator, so as to present saidsemi-finished lens to said machining means at said inclination angle (β), based on of said corrections to be produced.